There is, in a Patent Document 1, a description of a technique intended to realize high precision AD conversion by adding a ramp voltage having amplitude substantially equal to the minimum resolution of an AD converter circuit, performing an AD conversion operation plural times and averaging respective digital outputs by an arithmetic circuit having a larger number of digits than the number of digits of the AD converter.
In a Patent Document 2, a differential amplifier obtains the difference between an input analog signal and a reference signal corresponding to a conversion range, and an AD converter converts the difference into a digital signal. A control device that receives the digital signal therein controls the reference signal in response to a digital signal corresponding to the difference and generates an output digital signal in response to the digital signal and the conversion range. A resolution of conversion-range several times that of an actual AD converter can be obtained and a required resolution is obtained by way of setting the conversion range.